In practice not necessarily. In text, this would almost exclusively read as 'a child', except maybe when the sentence is something along the lines of 'Jullie hebben twee kinderen, maar ik heb een kind.' to strongly suggest otherwise. Context is key in these situations. To remove the ambiguity and force people to read it as a number, it's advisable to write it as 'één'.
You need to decide if you are talking to one person (je) or more than one (jullie). If the context doesn't indicate which, you can use either (with the correct form of the verb). In English 'you' is both singular and plural, but in older English the singular was 'thou'. Then people started using 'you' as the polite singular (as French does with vous) and in the end they were polite to everyone and thou dropped out of use!
I was told jullie was plural but with this it doesn't seem so. please explain.
Jullie is plural. Though the second person plural in English = you(http://www.grammar-monster.com/glossary/second_person.htm)
because in British English 'you' can be either singular or plural and we don't make any distinction. Originally 'you' was the plural form, then it was also used as the polite form when addressing one person, and eventually the more familiar 'thou' dropped out of use entirely (or almost).